1. Field of the Invention
The invention is directed to a novel electrophoretic fluid composition for improving temperature latitude and switching performance of an electrophoretic display, particularly at low operation temperatures.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side. There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972, filed Feb. 15, 2001, corresponding to US Publication No. 2002-0182544 (corresponding to WO02/65215), all of which are incorporated herein by reference. The improved EPD cells or microcups may be prepared by a photolithographic process or a microembossing method. In the microembossing method, a layer of thermoplastic or thermoset precursor composition coated on a substrate layer is embossed to form the microcups of well-defined shape, size and aspect ratio. The microcups are then filled with an electrophoretic fluid and sealed with a sealing layer. A second substrate layer is laminated over the filled and sealed microcups, optionally with an adhesive layer.
For all types of electrophoretic displays, the dispersion contained within the individual cells of the display is undoubtedly one of the most crucial parts of the device. The dispersion, as stated earlier, usually is composed of pigment particles dispersed in a dielectric solvent. The composition of the dispersion determines, to a large extent, the life time, contrast ratio, switching rate, response waveform and bistability of the device.
An improved dispersion composition is disclosed in a copending application, U.S. Ser. No. 10/335,051 filed Dec. 31, 2002 (corresponding to WO 03/57360). The dispersion is prepared by a microencapsulation process involving the use of a reactive protective colloid or dispersant to form part of the charged shell of pigment microparticles or microcapsules. In the process, an internal phase dispersion comprising primary pigment particles, such as TiO2 particles, a reactive monomer or oligomer and optionally a diluent is first emulsified into a continuous phase which comprises a reactive protective colloid in a fluorinated solvent or solvent mixture. During the emulsification step, a hard shell is formed around the internal phase particles as a result of the interfacial polymerization/crosslinking between the reactive monomer or oligomer from the internal phase and the reactive protective colloid from the continuous phase. The process allows the pigments to be density matched to the dielectric solvent. In addition, the reactive protective colloid is chemically bonded to the surface of the microcapsules, thus stabilizing the microcapsules and also improving the switching performance and longevity of the display. A charge controlling agent (CCA), particularly a reactive CCA as disclosed in the copending application, U.S. Ser. No. 10/335,210 filed Dec. 31, 2002 (corresponding to WO 03/58335), may also be incorporated into the shell of the microparticles to improve the particle size control and dispersion stability. As a result, the display performance such as longevity, image uniformity and switching performance are significantly improved.
Copending application U.S. Ser. No. 10/632,171, filed Jul. 30, 2003, discloses a method for improving the performance of an electrophoretic display which method comprises adding a fluorinated quaternary salt, fused ring or polynuclei derivatives or isomers thereof in either the internal phase or the continuous phase in a microencapsulation process for the formation of pigment-containing microcapsules or microparticles. The microencapsulation may be accomplished by direct or inverse emulsification. The method further comprises addition to the internal phase or continuous phase a fluorinated protective colloid, a second charge controlling agent, a second reactive monomer or oligomer or a combination thereof.
While the systems disclosed in the copending applications have provided a significant improvement in the display performance, the contrast ratio and response time, however, could be further improved, especially at low operating temperatures.